The invention relates to optical devices for filtering selected wavelengths of light using the mechanism of interference.
Interference filters use combinations of constructive and destructive interference to shape filter responses. Wavelengths subject to constructive interference pass through the filters, and wavelengths subject to destructive interference are blocked. The interference is created by overlapping different phase-shifted portions of the same beam. Examples include Fabry-Perot etalons, dielectric filters, and fiber Bragg gratings.
Fabry-Perot etalons use pairs of opposing partially reflective surfaces to produce multiple interference between reflected beam portions. However, the filter response is limited. Sinusoidal response curves are typical. Manufacturing is complicated by a requirement for precise alignment of the reflective surfaces.
Both dielectric filters and fiber Bragg gratings have alternating layers of high and low refractive index to produce a series of partial reflections that are offset by the spacing between the layers. Typically, the layers are spaced apart by one-quarter of the nominal wavelength of the filtered beam, which is difficult to hold for assembly of dielectric filters. Conventional manufacturing of the dielectric filters is limited to bulk optics, which are generally more costly than comparable integrated optics.
The index variation of fiber Bragg gratings is very low (e.g., 0.0001) so a very large number of layers are required to attenuate unwanted wavelengths. The alternating layers are made by exposing a photosensitive material to a standing wave. This limits the choice of materials to those which are photosensitive.
U.S. Pat. No. 4,715,027 to Mahapatra et al. discloses a multi/demultiplexer that can also be arranged as a filter. An echelon grating has reflective surfaces arranged in a staircase to reflect light back to a source at equally spaced frequencies. Although the filter can be manufactured as an integrated optic, its response is also limited. The filters must be cascaded in succession similar to a vernier to further refine the response.
My invention also filters selected wavelengths of light by dividing a input beam of light into two or more intermediate beams having different optical path lengths and by recombining the intermediate beams into an output beam that is modified from the input beam by interference between the intermediate beams. The optical path lengths of the intermediate beams can be varied by altering either the physical lengths of their respective paths or the refractive indices of the mediums in which they are conveyed.
My multi-path filter includes input and output optical pathways and an optical path length difference generator. One or more focusing optics can be used to couple the optical path length difference generator to the input and output pathways. For example, the input and output optical pathways can be aligned with an optical axis together with a pair of focusing optics and a spacer plate that functions as the optical path length difference generator. The first focusing optic collimates an expanding input beam emitted from the input pathway. The spacer plate has two or more intermediate pathways that divide the collimated beam into a corresponding number of intermediate beams having different optical path lengths. The second focusing optic converges and recombines the intermediate beams at a focus on the output optical pathway.
Another configuration of my multi-path filter includes adjacent input and output optical pathways that are offset from the optical axis. A single focusing optic cooperates with a spacer plate and a reflective optic for both collimating an input beam from the input pathway and focusing returning intermediate beams on the output pathway. With the addition of the reflective optic, the intermediate beams traverse the spacer plate two times, thereby doubling the differences between their optical path lengths. Alternatively, the reflective optic can be curved to also perform the functions of the focusing optic. A diverging beam from the input pathway is reflected on a return course through the intermediate pathways as a converging beam to the output pathway.
The intermediate pathways of the spacer plate can be varied in number, material, transverse area, and longitudinal length for achieving a desired spectral response. Optical path length differences between intermediate beams are created by varying the refractive indices of the materials forming the intermediate pathways, by varying their length, or both. The transverse areas of the intermediate pathways control the relative energies of the intermediate beams, and the number of different intermediate pathways controls the number of intermediate beams that can contribute to the desired spectral response. In general, the number of intermediate pathways is analogous to the number of slits in a conventional interference model.
In place of the spacer plate, a reflective stack can be used to-generate optical path length differences between the intermediate beams. Each layer of the reflective stack has a reflective coating that reflects a portion of the total light that is incident to the stack. The partial reflections provide a plurality of intermediate pathways that overlap spatially but separate the intermediate beams through different physical path lengths of at least one wavelength. The number, material, partial reflectivity, and longitudinal length of each layer can be controlled to adjust the spectral response.
My filter can be implemented in bulk optics, integrated optics, or in various hybrid combinations. For example, all of the elements can be constructed in planar technology. However, separately oriented elements are preferably used for more accurately reflecting light parallel to the other elements. My filter can also be incorporated within a single fiber. Two focusing optics and a spacer functioning as an optical path length difference generator are fusion spliced to join two ends of the fiber.
Materials within the spacer of the fiber or other implementation can be combined to exhibit different index characteristics with externally controlled conditions such as temperature, pressure, or electrical or magnetic fields. The controlled variation in the index difference between different intermediate pathways can be used for tuning the filter response to selected wavelengths.